Two questions though. Is The Return of Nagash a standalone novel or the opener of a new series? Like, I'm getting opener vibes, but the huge number of spoilers in the other thread is definitely odd.

No idea. I asked Josh that very question, also whether or not the End Times would be a series of novels dealing with different events like the Horus Heresy or if this was the start of a new series about Nagash in particular, and he said he couldn't say. The spoilers in the other thread are from the Warhammer: Nagash background book. The Return of Nagash is about Mannfred von Carstein and Arkhan the Black trying to resurrect Nagash.